The exact answer will depend on your system's extraction efficiency. For syrup extract, an estimate based on 70% mash efficiency is:

(pounds extract) * 1.33 = pounds grain

For dry extract it'd be:

(pounds extract) * 1.63 = pounds grain

The numbers are based on the potential yields. For syrup it's 36 ppppg (points/pound/gallon). For dry it's 44 ppppg. For grain it's 38 ppppg (at 100% efficiency). At 70% efficiency, your grain yield is about 27 ppppg (38 * 0.7 = 26.6). All you have to do is determine the ratios and you're set. From the syrup example, the ratio is the syrup yield to the grain yield (36/27=1.33). If you have a different extraction efficiency, just recompute the grain yield and then redetermine the ratio. At 80%, your grain yield is (38 * 0.8 = 30) and your conversion ratio would be (36/30=1.2).